In the following, the term “choke” relates to a configuration from one or several coils placed on a common core.
A step-up or step-down circuit refers to a circuit, which can increase or decrease a direct-current voltage. Step-up and Step-down circuits operate according to similar principles like power factor compensation filters and partially use the same components.
A power factor correction is imposed in Germany for electric loads over 75 watt since 1 Jan. 2001 by the electromagnetic compatibility norm (EMC). Power factor describes the rate between the value of the effective power and the apparent power. A value less than 1 means that the apparent power, which is drawn from the power grid, is larger than the effective power, so that the power grid is additionally loaded by the apparent power, which has to be provided and transported and which partially has to flow back through the power grids. Hereby greater losses occur in the grid and the grid has to be dimensioned larger than actually necessary. Power factor correction filters make sure that the power factor is as close as possible to 1, i.e. only pure effective power is drawn from the power grid. In an active power factor correction (PFC) the drawn current is readjusted to the time dependent sinus shape voltage of the power grid.
A central component of step-up, step-down circuits and of PFC is a choke, which is in principle used to temporarily store Energy and release it on requirement. The following explanations confine on the use of the choke in PFC filters. However, similar reasoning is also true for step-up and step-down circuits.
A switch connected downstream of the choke which can adjust the coil output to a reference potential, is opened and closed by a controlling device so as on the one hand to deliver sufficient power to an electric load, but on the other hand so that the current of the grid voltage curve drawn from the grid is in-phase.
In a further development the input power voltage is divided between two coils which can be operated independently from one another. In general the switches are operated inverse to one another, i.e. if one switch is opened, the other switch is closed. In such an “interleaved” operational mode a choke branch (master) is directly controlled by the regulation circuit, i.e. the switching times for the choke are directly controlled by the regulation. The second choke branch (slave) generally follows the master with a phase shift of 180 degrees. Such an interleaved working arrangement has the advantage, that a more efficient power factor correction can be achieved. Since each choke has to cope with only half of the output power, smaller components can be dimensioned, so as to improve the power loss and heat generation and allow for smaller PFC-circuits. It is to be noted, that a correct functioning is possible also at other phase shifts <180°. That is, in general, the phasing can be variable. However, the majority of applications operate with a phase shift of 180°.
Active PFC circuits usually consist of a rectifier with a step-up convertor directly attached downstream with a coil and a switch, which charges a large capacitor to a voltage above the peak voltage of the grid network alternating current. FIG. 1A schematically shows the principles of a step-up circuit in interleaved technology. At the input the input voltage VIN is applied to the two choke coils L1 and L2 and the input current IIN is divided between the two chokes. At the output of each coil or choke L1 or L2 a switch S1 or S2, respectively, can set the output L1 or L2 controlled by a regulation circuit (not shown) to a reference potential. The outputs of coils L1 and L2 are connected through a diode to the capacitor COUT, which, in interaction with the coils L1 and L2 increases the voltage (step-up circuit) and smoothes the voltage, so that it can be delivered to the load resistance RLOAD.
The opening and closing times of switch S1 are set by a controller (master), which ensures that on the one hand the load RLOAD is provided with sufficient current IOUT and on the other hand the input voltage IIN is following in-phase the input voltage VIN. The switch S1 follows the switch S1 phase shifted by 180 degrees (slave). This causes in principle a pulse width modulation of the input current, in which the pulse width is controlled by a controller. FIG. 1B shows the switching characteristics of the switches S1 and S2. The time in which switch S1 is closed is denoted as Ton and is variable according to the controller. In the time in which switch S1 is closed, switch S2 is opened (180 degree phase shifted). The overall time, which consists from the sum of the time Ton, in which the switch S1 is closed and the time Toff, in which the switch is opened, is denoted as period T and is constant. The duty cycle D=Ton/T is variable and dependent on the controller. In FIG. 1b a constant duty cycle D of 0.5 is shown.
FIG. 1C shows the currents I1 and I2 through the coils L1 and L2. The current I1 through coil L1 consists from a direct current component Idc1 and a ripple component Iac1, generated by the switching processes. Accordingly, the current I2 through the coil L2 consists from a direct current component Idc2 and a ripple component Iac2 (alternating current component caused by switching processes). Since the switches are connected with a phase shift by 180 degrees, the phase shift between Iac1 and Iac2 is 180 degrees. On the capacitor COUT the currents I1 and I2 are added. I.e. the complete direct current component results in Idc=Idc1+Idc2. From Idc1=Idc2=IIN/2 follows for Idc that Idc=IIN. For the complete ripple current component (alternating current component) follows Iac=Iac1−Iac2, since Iac1 and Iac2 are phase shifted by 180 degrees. This, however, is only true for a duty cycle of D=0.5, i.e. for ton=toff. I.e. for a duty cycle of D=0.5 the ripple current components mutually compensate. At different duty cycles the ripple current components do not precisely compensate each other. In any case, on the whole, in the interleaved design the ripple current component is reduced giving a smoother current curve.
It should be noted, that at a phase shift of 180° the ripple current maximum in the middle leg is reached at a duty cycle of D=0.5. The interleaved choke, however, also functions at other phasings <180°. Hereby only the duty cycle D, at which the maximum of the alternating current ripple occurs, is shifted. I.e. that in general the phasing can be variable. However, the majority of applications operates at a phase shift of 180°.
Chokes for use in interleaved step-up circuits and PFC steps are known from state of the art. In the simplest case two coils are wound on a common core, like for example shown in U.S. Pat. No. 6,362,986 B1 of the Volterra company. FIG. 2 schematically shows the coil arrangement of this patent with switches 40 for the interleaved operation mode. The two coils 20 and 30 are arranged on a common ring-shaped core 10, i.e. the pair of coils 20, 30 works with a strong magnetic coupling, similar to a transformer. Since the magnetic flows from the coils sum up, the core geometries are correspondingly large, so as to reach a high magnetic conductivity and at the same time not to stress the core up to the saturation magnetization.
U.S. Pat. No. 8,217,746 B2 describes a further development of a choke coil for interleaved PVC circuits, in which the coil core for the two coils is designed such that the two coils are only weakly magnetically coupled. FIG. 3 shows a schematic view of the coil- and core configuration of U.S. Pat. No. 8,217,746 B2. The core consists from two E-shaped parts 110 and 120, which are separated from one another by an I-shaped part 130. The coils 20 and 30 are wound on the middle legs of the E-shaped parts 110 and 120. Since the magnetic flows Φ1 and Φ2 in the coils 20 and 30 from the middle legs of the E-shaped parts divides between the lateral legs of the E-shaped parts, the cross section A2 of the lateral legs can be half the size of the cross section A1 of the middle legs. Since the coils 20 and 30 are wound or connected in phase opposition, the direct current components of the magnetic flows Φ1 or. Φ2 of the coils 20 and 30 in the I-shaped portion of part 130 extent compensate one another to a large, so that the cross section of the I-shaped part 130 can be designed smaller than the cross section A1 of the middle legs of the E-shaped parts 110 and 120. By connecting the two E-shaped parts 110 and 120 and of the I-shaped part 130 air gaps 140 are formed at the joints.